1. Field of the Invention
The invention relates to the use of an iron-based alloy which includes nitrogen in amounts to harden the alloy for use in machine parts which are subject to severe stresses caused by sliding friction. Some of the preferred uses for the inventive alloys are in plunger pistons and piston rings.
2. Description of Background Information and Relevant Information
Machine parts that are exposed to high or primary stress from sliding friction, in particular superficial friction, are predominantly made of cast materials. Cast parts have the advantage of undergoing an essentially isometric change in shape upon a change of temperature (i.e., they expand and contract isometrically) and therefore are not abrasive in practical use. This is due to the fact that the cast parts have a largely random microstructure and a homogeneous microscopic hard-particle distribution, such as, e.g., carbide distribution, and an equally homogeneous soft matrix configuration. This kind of microstructure is especially advantageous at frictional surfaces, and it decreases both the forces of friction and abrasion of material, especially if the surface is inadequately or poorly and/or incompletely coated with lubricant. Slide bearings, cylinder bushings, piston rings and similar machine parts that are exposed to severe stress from sliding friction are therefore primarily produced from cast or sintered materials, especially iron-based alloys.
When the parts are additionally subject to corrosive stresses, as in the case, for instance, with plunger pistons or piston rings in internal combustion engines, as a result of positive displacement media, condensates or the like, it is necessary to provide high chromium content in the alloy. However, the other properties of the material that are needed for an appropriate function of the part must not be disadvantageously affected by the inclusion of chromium.
For machine parts exposed to sliding superficial friction, and especially for piston rings, the attempt has already been made to use deformed material, in other words, materials that have a band-like deformation structure. These deformed parts (i.e., parts made from deformed material), have a microtexture designed so that the parts expand and contract differentially with temperature changes, to better meet the demands made of them in practical use. The manufacture of deformed parts, however, requires a number of complicated process steps in order to establish a required form or size for the particular use and the desired material properties.
Piston rings of deformed materials, such as an alloy of DIN material number 1.4112 or material number 1.2361, can be produced by rolling and optionally drawing a profile wire. Then, the wire is quenched, tempered and wound or cold-deformed into a spiral. After required annealing, it is advantageous to wind up, or further deform the profile wire in order to at least partially reduce the cold deformation stresses or solidifications of this spiral preform, to make a piston ring spiral, optionally with a smaller radius than that of the particular piston ring diameter which the ring is designed for.
Preferably after a further stress-relieving treatment or annealing, the spiral, optionally made in a nonround form, is cut open essentially in the direction of the generatrix, and the individual rings thus formed are machined. A nonround form of the piston rings in the stress-relieved state can be chosen so that in their work position in the cylinder, they are pressed radially inward on all sides, and an essentially equally high specific contact pressure of the outer ring surface against the cylinder wall will be attained. The piston rings should have a hardness of 35 HRC or 45 HRC and especially above 40 HRC.
If after rolling and optionally cold-drawing to a desired cross-sectional profile, the precursor material is then thermally quenched and tempered to a requisite high hardness, for instance in a once-through or continuous operation, then the thus heat-treated material has low bending strength, and as a result cracks can sometimes occur when the profile wire is wound into a spiral. The highest possible demands are in fact made of the strength or cold deformation capacity of the material, and these demands are also affected by the profile geometry and by the form of the spiral. Often, expensive special annealing processes and/or annealing operations must be carried out in order to attain a high level of uniformity in this material property.
Before rewinding into a spiral form oriented to the particular piston diameter is done, however, an expensive annealing of the piston ring precursor material that relieves stress and increases its strength must be provided. Otherwise the cold deformation capability of the material would be exhausted, and this would inevitably lead to breakages of material.
To overcome these disadvantages, the attempt has already been made to reduce the carbon concentration of the alloy and thus to improve the bending strength of the quenched and tempered material. However, a reduced carbon content lessens the hardness and makes the usage properties, especially the sliding properties, of the piston rings substantially worse. Accordingly, this attempt has been generally unacceptable in practice. The attempt was also made to increase the bending strength of the material by alloying it with cobalt. This technique has long been known and used in the preparation of high-speed steel. Although some improvement was attainable as a result, nevertheless in many cases the increase in strength values was not adequately great, and the high cost of the cobalt alloy metal made this approach commercially disadvantageous as well. In addition, comparatively greater abrasion in deformed alloys was ascertained.